The present invention relates generally to gas turbine control. More particularly, the invention relates to a method and system for providing advance warning or avoidance of lean blowouts for the combustion system of a gas turbine, especially directed to a Dry Low NOx (DLN) combustor.
Industrial and power generation gas turbines have control systems with controllers that monitor and control their operation. These controllers govern the combustion system of the gas turbine. To minimize emissions of nitric-oxides (NOx), DLN combustion systems have been developed and are in use. Control scheduling algorithms are executed by the controller to operate DLN combustion systems. Conventional DLN algorithms receive as inputs measurements of the exhaust temperature of the turbine and of the actual operating compressor pressure ratio. DLN combustion systems typically rely solely on the turbine exhaust temperature and compressor pressure ratio to determine an operating condition, e.g., turbine exhaust temperature, of the gas turbine.
FIG. 1 depicts a gas turbine 10 having a compressor 12, a combustor 14, and a turbine 16 coupled to the compressor and a control system or controller 18. An inlet 20 to the compressor feeds ambient air and possibly injected water to the compressor. Air flows through the inlet 20 into the inlet guide vanes 21 of the compressor. An exhaust duct 22 for the turbine directs combustion gases from the outlet of the turbine through ducts having, for example, emission control and sound absorbing devices. The turbine may drive a generator 24 that produces electrical power and supplies the electric power through a breaker 25 to an electrical grid 27.
The operation of the gas turbine may be monitored by several sensors 26 detecting various conditions of the turbine, generator and environment. For example, temperature sensors may monitor ambient temperature surrounding the gas turbine, compressor discharge temperature, turbine exhaust gas temperature, and other temperature measurements of the gas stream through the gas turbine. Pressure sensors may monitor ambient pressure, and static and dynamic pressure levels at the compressor inlet and outlet, and turbine exhaust, as well as at other locations in the gas stream. Further, humidity sensors, e.g. wet and dry bulb thermometers, measure ambient humidity in the inlet duct of the compressor. The sensors 26 may also comprise flow sensors, speed sensors, flame detector sensors, valve position sensors, guide vane angle sensors, or the like that sense various parameters pertinent to the operation of gas turbine 10. As used herein, “parameters” and similar terms refer to items that can be used to define the operating conditions of turbine, such as temperatures, pressures, and flows at defined locations in the turbine that can be used to represent a given turbine operating condition.
A fuel control system 28 regulates the fuel flowing from a fuel supply to the combustor 14, the split between the fuel flowing into various nozzles and the fuel mixed with air before flowing into the combustion zone, and may select the type of fuel for the combustor. The fuel control system may be a separate unit or may be a component of a larger controller 18.
The controller may be a General Electric SPEEDTRONIC™ Gas Turbine Control System. The controller 18 may be a computer system having a processor(s) that executes programs to control the operation of the gas turbine using sensor inputs and instructions from human operators. The programs executed by the controller 18 may include scheduling algorithms for regulating fuel flow to the combustor 14. The commands generated by the controller cause actuators on the gas turbine to, for example, adjust valves between the fuel supply and combustors that regulate the flow and type of fuel, inlet guide vanes 21 on the compressor, and other control settings on the gas turbine.
The controller 18 regulates the gas turbine based, in part, on algorithms stored in computer memory of the controller. Fuel and air are combined in a combustion process in gas turbine engines. To control the production of oxides of nitrogen (NOx) in this process, combustion flame temperature and fuel mixing must be tightly maintained. Fuel and air can be premixed uniformly to avoid localized areas of high combustion temperature and the engine can be operated below certain temperatures to avoid production of unacceptable amounts of NOx. These algorithms enable the controller 18 to maintain the NOx and carbon monoxide (CO) emissions in the turbine exhaust to within certain predefined limits, and to maintain the combustor firing temperature to within predefined temperature limits. The algorithms include parameters for current compressor pressure ratio, compressor discharge temperature, ambient specific humidity, inlet pressure loss and turbine exhaust back pressure.
Typically, significant margin exists on combustion systems in that transient under-fire events have no significant negative impact. However on advanced ultra low emissions combustion systems, the margins are much tighter. Transient under-fire can result in combustion dynamics or a loss of flame. Combustion dynamics within the combustor are known to damage hardware. Loss of flame in a combustion can creates high exhaust temperature spreads. The plugs are fired returning the machine to Lean Lean, a high emissions mode of operation. A unit trip can also occur on high spreads.
Gas turbines with dry low NOx combustion systems operate at very lean fuel/air (F/A) ratios closer to a Lean Blowout (LBO) boundary in order to maintain low NOx emissions. F/A ratios leaner than the LBO boundary value can result in blowout of the flame. Further, can-to-can F/A ratio variability results in cans having F/A ratios closer to the LBO boundary that are more prone to blowout than cans that are operating at a larger margin from the LBO boundary. A blowout in one can sometimes leads to blowout in several adjacent cans, which can eventually trigger a turbine trip. Turbine trips due to LBO can be costly. Revenue can be lost during downtime due to trips and physical damage can be inflicted on the combustion components due to the blowout.
Lean blowout or weak extinction is the point at which the mixture of fuel and air is no longer flammable. For premixed multi-nozzle systems, weak extinction can be defined as the point at which there is a significant drop in the combustion efficiency and or complete extinction of the flame.
Prior art apparatus and methods by Norman et al. (U.S. Publication 2005/0278108) for predicting lean blowouts include extracting a plurality of tones in pressure signals representative of pressure within monitored combustion cans, tracking a frequency of a hot tone in each monitored cans, and utilizing extracted tones and the tracked frequency to determine a probability of an LBO. Taware et al. (U.S. Publication 2006/0042261) incorporates use of pressure signals and/or flame detector signals to determine the presence of an LBO precursor and the probability of an LBO, and to initiate a gas turbine response to avoid an LBO event. However, these systems require special transducers and instrumentation and may be more appropriate for testing and inspection purposes than for continuous online operation.
When a lean blowout occurs, either high spreads or the reduction in combustion reference temperature will fire the spark plugs in order to relight the unlit cans. The unit will revert to Lean Lean or Extended Lean Lean which is a higher emissions mode of operation. In order to return to the lower emissions mode, which utilizes premixed fuel and air, the unit must be unloaded significantly to clear a software latch resetting premixed mode. If the original issue that caused the blowout has not been connected by adjusting controls algorithms, the unit may be unable to transfer back into the premixed fuel mode and stay there. This can result in a forced shutdown as customers are often forbidden to operate in high emissions modes for any length of time by government regulations.
Loss of flame in a combustion can creates high temperature spreads, but because of the lag inherent to temperature measurement, the spark plugs can be triggered before the temperature spread can be used to identify the blowout. The need is to identify the blowout as an unintentional transition and not the result of normal power transitions.
Accordingly, new control algorithms are required to identify LBO events and transiently position the gas turbine unit to prevent combustion dynamics or loss of flame in subsequent operation while conforming to strict emission requirements.